Generally, when a bulldozer blade is raised or lowered with the bulldozer blade tilt-operated, i.e., with either one of the right and left ends of the bulldozer blade tilted downward, this causes one of the blade lift cylinders to quickly arrive at its stroke end, as a result of which an extensive force is exerted upon only the other blade lift cylinder. This sometimes brings about an undesirable situation in which there occurs an extensive torsional stress in a structural member (e.g., a radiator guard) for supporting the blade lift cylinder in question or in the vehicle body frame. For this reason, structural members for supporting blade lift cylinders have been made solid enough to withstand an extensive torsional stress.
In order to address with the above-described problem, there has been reported some techniques. For example, Japanese Utility Model Publication (KOKAI) Gazette No. 4-37650 (1992) discloses a system, wherein the difference between stroke values of the right and left tilt cylinders is used to detect a blade lateral tilt angle. When the blade lateral tilt angle exceeds an allowed value, the tilt limit valve is actuated for the avoidance of interference between the bulldozer blade and the vehicle body. Japanese Patent Publication (KOKAI) Gazette No. 63-63830 (1988) shows a system as another related technique. In accordance with this system, when the bulldozer blade in a tilt state is in the vicinity of the limit of ascent/descent thereof, it is arranged such that the flow rate of pressure oil, which is pressure-supplied to the lift cylinders for raising and lowering the bulldozer blade, is gradually diminished for the avoidance of impact due to the lift cylinders arriving at their stroke ends.
However, the above-described method, in which structural members for supporting the blade lift cylinders are reinforced in structural strength, is problematic in that the bulldozer body becomes heavier in weight and in addition, manufacturing costs increase.
One of the above-described publications is intended for the prevention of interference between the bulldozer blade and the vehicle body at tilt operation time, whereas the other publication is intended for the relaxation of impact that occurs when the lift cylinders reach their stroke ends. Consequently, these techniques fail to provide any effective solution essential to a reduction in torsional force occurring in the body frame when the bulldozer blade is operated to ascend or descend while being tiltoperated.
In consideration of the above-described drawbacks with the prior art techniques, the present invention was made. Accordingly, an object of the present invention is to provide an improved hydraulic system for use in a bulldozer capable of preventing the occurrence of exertion of an extensive torsional force upon the vehicle body frame due to the operation of the blade lift cylinders.